Ok. I've asked before, I think, and I've researched it extensively, and I'm more confused about this now than I was when I first asked. What exactly do I look for? I have a Crown Graphic 4x5 with a 135mm/4.7 Optar. It has nothing to screw a filter in, so I guess it has to be a slip on affair. The size seems to be just a touch bigger than a 40.5mm filter. If I get something that says Series VI, will that automatically fit? Is there anything special I need to get with it? Any help you can provide would be greatly appreciated. I do have all the 52mm screw-on filters I need (for my Nikon outfit), and it would be great if I could find some adapter that would allow me to use these on the Graphic.

Seems you've got all the info. Although I thought the 135 Optar took the 31.5mm slip-on? But I may be thinking of the 90mm? A half mm will make a big difference in how the slip-ons fit, so measure carefully!

They used to make screw-in adapters for the series filter holders for 'standard' screw-in filters. They can be found if you look and wait. I can adapt my series VI and VII al the way up to my 77mm filters if I need to...

What you need specifically is a Series VI slip-on filter holder in the proper size for your lens, which may be the 40.5mm you say.

Then you need one or more screw-in adapters to convert the Series VI filter holder to your 52mm filters. Simple as that...

But good luck finding them. It's getting harder all the time. But I admit that I haven't searched for any in over a year or so...

And maybe someone remembers better than me, but isn't the Series VII the standard 52mm filter thread? Or was it 55mm??? It's been decades since I used a lens with a 52mm filter thread and I don't remember... I mention because a Series VI to Series VII adapter may be easier to find...